Flex bit sizes: flexible installer bit size chart
A flex bit (flexible installer bit, cable bit, or bell hanger's flexible shaft bit) is a long spring-steel drill an electrician feeds through a small wall opening to bore studs, top plates, and headers from inside the cavity, then pulls wire back through the hole using the fish eye in the tip. Search results for sizes are almost entirely first-party catalog tables, so this page assembles them into one chart: every head diameter, shank diameter, and overall length the major brands actually sell, with part numbers and the extension compatibility rules that decide which bits can grow longer [1][3][5].
The size grid at a glance
Two numbers define a flex bit: the head (cutting) diameter and the shank (shaft) diameter, and they are not independent. Every major brand builds the small family on a 3/16 in shank and the large family on a 1/4 in shank:
| Head diameter | Shank diameter | Typical lengths (in) | Works with extension family |
|---|---|---|---|
| 3/8 in | 3/16 in | 18, 24, 36, 54, 72 | 3/16 shank (Klein 53723 and equivalents) [1][3] |
| 1/2 in | 3/16 in | 18, 24, 36, 54, 72 | 3/16 shank [1][13] |
| 9/16 in | 3/16 in | 18, 24, 36, 54, 72 | 3/16 shank (Klein limit: 9/16 in and smaller) [1][10] |
| 3/4 in | 1/4 in | 18, 24, 36, 54, 72 | 1/4 shank (Klein limit: 3/4 in and larger) [1][9] |
| 1 in | 1/4 in | 18, 24, 36, 54 (72 in ICS only) | 1/4 shank [1][8] |
The 3/16 / 1/4 shank split is the spec that matters most when buying, because it decides which extensions and replacement bits pair with your existing kit; the section below spells out the rules. One gap to know: Eagle Tool stops its 1 in head at 54 in, and no 1 in x 72 in auger flex bit exists in the Eagle, Klein, or LH Dottie lines; ICS is the one maker with a 1 in x 72 in flexible shank ship auger [1][7].
Eagle Tool X-FLEX and screw point part numbers
Eagle Tool US sells the fullest grid in the category, in two tip styles with the same numbering scheme: EA (auger style, X-FLEX and classic) and ESP (screw point). Head and shank pairings below are from the manufacturer's own tables [1][3].
| Head x shank (in) | 18 in | 24 in | 36 in | 54 in | 72 in |
|---|---|---|---|---|---|
| 3/8 x 3/16 | EA37518 | EA37524 | EA37536 | EA37554 | EA37572 |
| 1/2 x 3/16 | EA50018 | EA50024 | EA50036 | EA50054 | EA50072 |
| 9/16 x 3/16 | EA56218 | EA56224 | EA56236 | EA56254 | EA56272 |
| 3/4 x 1/4 | EA75018 | EA75024 | EA75036 | EA75054 | EA75072 |
| 1 x 1/4 | EA10018 | EA10024 | EA10036 | EA10054 | not made |
The ESP screw point series (ESP37518 through ESP75072) mirrors the EA grid size for size, and the ET X-FLEX notched-shank versions use the same part numbers with the shank notch that keeps set screws from slipping [3][4]. Second surfaces for the part matrix: Grainger lists EA37536 and EA75054 with the same head and shank figures [2][6], Menards lists the EA50072 X FLEX with the 3/16 in notched shank [4], and Lowe's and Walmart carry the 3/8 x 36 and 1 x 54 listings [5][8]. Eagle's own extension pair is explicit about the split: EX18736/EX18752 extensions (3/16 in body) fit the 3/16 shank bits [4].
Klein Tools flex bit augers and extensions
Klein's flex augers sit at the professional end: 54 in standard, 72 in on two SKUs, each with a fish eye hole on both ends so wire can be pulled from either direction. Head sizes map to extension compatibility in Klein's own spec blocks [9][10][13].
| Cat. No. | Head diameter | Length | Pairs with extension |
|---|---|---|---|
| 53716 | 3/8 in | 54 in | 53723 (3/16 shank) [9][11] |
| 53717 | 3/8 in | 72 in | 53723 [10][14] |
| 53718 | 9/16 in | 54 in | 53723 (Klein's stated limit: 9/16 in and smaller) [13][15] |
| 53719 | 3/4 in | 54 in | 53722 (1/4 shank) [12][16] |
| 53720 | 1 in | 54 in | 53722 [12][17] |
| 53751 | 3/4 in | 72 in | 53722 [12][17] |
Extension compatibility: the rule nobody prints on the box
Extensions are sold by shank diameter and connection diameter, and mixing families is the classic mistake. Klein publishes the split as a spec: the 53723 extension (3/16 in shank, 7/16 in connection diameter) is for flex bits 9/16 in and smaller, while the 53722 extension (1/4 in shank, 5/8 in connection diameter) is for flex bits 3/4 in and larger [9][10]. That aligns exactly with how the bits themselves are shanked: every 3/8, 1/2, and 9/16 head rides a 3/16 shaft and every 3/4 and 1 in head rides a 1/4 shaft across Eagle and ICS tables too [1][7]. Milwaukee states the same rule for its system: the 3/16 in x 52.5 in extension 48-13-8118 is compatible with all cable bits with 3/16 in shanks [18]. ICS's flexible shaft extensions chain to each other with two set screws for extra-long reach, sized 3/16 in body for the 3/8 head and 1/4 in body for 1/2 through 1 in heads [7].
| Extension | Body / shank | Connection diameter | Fits heads | Length |
|---|---|---|---|---|
| Klein 53722 | 1/4 in | 5/8 in | 3/4 in and larger | 54 in [9][16] |
| Klein 53723 | 3/16 in | 7/16 in | 9/16 in and smaller | 54 in [10][15] |
| Milwaukee 48-13-8118 | 3/16 in | set screw, 3/32 hex key | 3/16 in shank bits | 52.5 in [18][19] |
| ICS BHFSX032X54 | 1/4 in | set screw, chaining | 1/2 in head (3/16 body = 3/8 head) | 54 in [7][20] |
LH Dottie Flex-A-Bit and ICS flexible shaft sizes
LH Dottie's Flex-A-Bit line covers five head sizes but only the two long lengths, with a permanently brazed self-feeding point and a fish hole in the tip [5]. ICS Cutting Tools sells flexible shank ship auger bellhanger drills in 36, 54, and 72 in with its own numbering (BHFSSA) plus economy flexible shaft extensions (BHFSX); ICS rows are corroborated on ICS documents only (catalog page 107B and the item listing index), disclosed here as manufacturer-sourced [7][20].
| Brand and series | 3/8 in | 1/2 in | 9/16 in | 3/4 in | 1 in |
|---|---|---|---|---|---|
| LH Dottie Flex-A-Bit, 54 in [5][21] | FB3854 | FB5054 | FB9654 | FB7554 | not made |
| LH Dottie Flex-A-Bit, 72 in [5][22] | FB3872 | FB5072 | FB9672 | FB7572 | not made |
| ICS BHFSSA auger, 36 in [7][20] | 024X36 (built to order) | 032X36 | not made (5/8 in instead) | 048X36 | 100X36 (built to order) |
| ICS BHFSSA auger, 54 in [7][20] | 024X54 | 032X54 | not made (5/8 in instead) | 048X54 (built to order) | 100X54 |
| ICS BHFSSA auger, 72 in [7][20] | 024X72 | 032X72 | not made (5/8 in instead) | 048X72 | 100X72 |
Two ICS quirks worth knowing before you order. ICS sizes its flexible shank augers by a different head ladder: 3/8, 1/2, 5/8, 3/4, 1 in, with 5/8 replacing the 9/16 that electrician brands use, all on 3/16 in (3/8 head) or 1/4 in (1/2 in and up) shanks [7]. And three cells in its table print a dash instead of a price (3/8 x 36, 3/4 x 54, and 1 x 36, part numbers 024X36, 048X54, and 100X36): the part exists in the catalog but is built to order rather than stocked [7].
Milwaukee rounds out the mainstream options with cable bits on the 3/16 shaft family: 48-13-8250 (1/2 in x 54 in, 3/16 in shaft diameter printed on the product page) and the 72 in 48-13-8350 [18][19]. Milwaukee does not publish a 1/4 shank flex auger family; for 3/4 and 1 in heads the electrical trade standard is Klein or Eagle.
Auger vs twist flex bits, fish holes, and steering
Nearly every flex bit sold for electrical work is auger style: a self-feeding screw point pulls the bit into the wood and the deep flute clears chips, which is what you want when the bit is 4 or 6 feet from you inside a dark cavity [1][18]. Twist style flexible bits exist (ICS's HSS flexible shank bellhanger drills, page 107 of its catalog) and double as their own fish tape, trading chip speed for a lower price and a tip that can be dressed like an ordinary drill bit [7]. The fish hole is common to both: a hole through the tip (Eagle puts accommodation holes in the flute and the shank both; Klein puts a fish eye on either end so you can pull from either direction) that you tie wire to before withdrawing the bit [1][13].
Steering is the technique that makes the long lengths usable. A 54 or 72 in bit will droop under its own weight, so electricians use a placement or bending tool: Klein's 53715SEN and ICS's FSBT both hold the shaft near the head so you can aim and start the screw point on target, then steer it around obstructions mid-cavity [9][7]. The same memory-spring steel that lets the shaft bend is what snaps it back straight, ICS's stated design intent [7].
For the sizing logic around the holes themselves, our bell hanger bit size chart covers the rigid end of this family (12 to 42 in lengths, LH Dottie's BLH series) plus the code rules: 3/8 in for a single NM cable, 1/2 in for multiple, 1-1/4 in edge distance with nail plates. The auger bit size chart covers rigid ship augers, and wood boring bits compared places flex bits among spade, Forstner, and hole saw options. For drilling speed by diameter, the drill speed chart applies inside walls too: run a flex bit slow enough to control, because a grabbing screw point torques the drill hard.
Quick answers
- What sizes do flex bits come in? Heads of 3/8, 1/2, 9/16, 3/4, and 1 in on 3/16 or 1/4 in shanks, in 18, 24, 36, 54, and 72 in lengths. The full grid exists only across brands; the part number tables above show exactly who makes what.
- Which extension fits which bit? 3/16 in shank extensions (Klein 53723, Milwaukee 48-13-8118) for heads 9/16 in and smaller; 1/4 in shank extensions (Klein 53722) for 3/4 in and 1 in heads. Klein publishes the split as a spec on both extension pages.
- Is there a 1 in x 72 in flex bit? Only from ICS (BHFSSA100X72). Eagle stops at 54 in, and Klein and LH Dottie do not offer a 1 in head at 72 in.
- Why 9/16 in? It is the electrician's between-size: bigger than 1/2 in for doubled cable runs, small enough to stay on the lighter 3/16 in shank and its extension family. Klein, Eagle, and LH Dottie all carry it; ICS substitutes 5/8 in.
Related guides
Also on this site: the bell hanger bit size chart for the rigid 12 to 42 in wire-hole bits and code hole-sizing rules, the auger bit size chart for rigid ship augers, wood boring bits compared for the whole wire-hole family, and the drill speed chart for RPM by diameter. For choosing steel and coating instead of size: drill bits by material.
Sources
- Eagle Tool US, Auger Style Installer Bits (wood applications): full EA table, head 3/8 to 1 in, shank 3/16 and 1/4 in, OAL 18 to 72 in, fish accommodation holes in flute and shank, three flat nonslip grip: eagle-tool.us
- Grainger, EAGLE TOOL US Auger Style Cable Drill Bit 33GM38 = EA37536, 3/8 in bit size, 36 in overall: grainger.com
- Eagle Tool US, X FLEX Installer Drill Bit Auger Style: same EA grid as ETX FLEX with notched shank, 1 in tops out at 54 in: eagle-tool.us
- Menards, Eagle Tool US X Flex 1/2 in x 72 in Screw Point Auger Drill Bit EA50072XFlex with 3/16 in notched shank: menards.com
- LH Dottie, Flex-A-Bit product class table: FB3854/FB3872, FB5054/FB5072, FB9654/FB9672, FB7554/FB7572, 54 and 72 in only, no 1 in: lhdottie.com
- Grainger, EAGLE TOOL US Auger Style Cable Drill Bit 33GM44 = EA75054, 3/4 in bit size, 54 in overall, 1/4 in shank: grainger.com
- ICS Cutting Tools, catalog page 107B: flexible shank ship auger bellhanger drills BHFSSA 3/8 to 1 in by 36/54/72 in with 3/16 and 1/4 shanks, BHFSX flexible shaft extensions, FSBT bending tool, memory retention shank: icscuttingtools.com PDF
- Lowe's, Eagle Tool US Auger style Flex 1 in x 54 in Cable Installer Bit (EA10054): lowes.com
- Klein Tools, Flex Bit 54-Inch Extension 1/4-Inch Shank 53722: for flex bits 3/4 in and larger, connection diameter 5/8 in: kleintools.com
- Klein Tools, Flex Bit 54-Inch Extension 3/16-Inch Shank 53723: for flex bits 9/16 in and smaller, connection diameter 7/16 in: kleintools.com
- Klein Tools, Flex Bit Auger 3/8-Inch x 54-Inch 53716: bit diameter 3/8 in, For Use With 53723: kleintools.com
- Klein Tools, Flex Bit Auger 1-Inch x 54-Inch 53720: bit diameter 1 in, For Use With 53722: kleintools.com
- Klein Tools, Flex Bit Auger 9/16-Inch x 54-Inch 53718: bit diameter 9/16 in, fish eye hole on either end, For Use With 53723: kleintools.com
- Klein Tools, Flex Auger Bit with Screw Point 3/8 x 72-Inch 53717: bit diameter 3/8 in, 72 in, For Use With 53723: kleintools.com
- Home Depot, Klein Tools 9/16 in. x 54 in. Flex Auger Bit 53718: homedepot.com
- Acme Tools, Klein 53722 Flex Bit 1/4 in Shank 54 in Extension: for use with flex bits 3/4 in and larger, connection diameter 5/8 in: acmetools.com
- Klein Tools, Flex Bit Auger 3/4-Inch x 72-Inch 53751: bit diameter 3/4 in, 72 in, For Use With 53722: kleintools.com
- Milwaukee Tool, 1/2 in x 54 in Cable Bit 48-13-8250: 1/2 in bit diameter, 54 in length, 3/16 in shaft diameter, dual pull-through holes, auger-style flute: milwaukeetool.com
- Home Depot, Milwaukee 1/2 in. x 72 in. Cable Bit 48-13-8350: homedepot.com
- ICS Cutting Tools, Item-Listings.pdf index: BHFSX and BHFSSA mapped to catalog page 107B (second ICS document corroborating the series and page): icscuttingtools.com PDF
- Graybar, LH Dottie FB5054 Flexible Bit 1/2 x 54 in: spring steel shaft, self-feeding brazed point, tapered back, hole in tip for wire pulling: graybar.com
- Elliott Electric Supply, LH Dottie FB7554 3/4 x 54 Flex-A-Bit: bit diameter 3/4 in, overall length 54 in: elliottelectric.com